Hello Fred,
In YoWindow with my station meteorological data (http://vyronas.meteoclub.gr), the pressure value is not displayed, but instead the display NaN hPa, in recent days.
look at the link http://vyronas.meteoclub.gr/yo1.jpg for a picture.
I use Visual Weather Station program and the file yowindow.htx
creates the file yowindow.xml, in which the pressure value is there, but does not appear in yowindow.

Thank you for replying in English.
Now I understand your problem better.
Unfortunately I don't know anything about Visual Weather Station.

However I was thinking that recently YoWindow has changed it's displaying of the pressure.
Before it was displayed standard at location level.
Now it's displayed standard at sea level.
But it's also possible to make a choice of which level you want it to be displayed.

Could you go to http://yowindow.com/weatherwidget.php and try your location with the parameters you want to be displayed,
including the pressure at location- or sea level and copy the code into your website again?

I did just found that out too after trying it myself.
This needs further investigation and maybe help from the developer.
I've asked him for his help.
Since he's very busy it may take some time.
I will get back to you as soon as I've got news.